The present invention relates generally to a taking zoom lens system, and more particularly to a compact zoom lens system designed to be used on 35-mm silver salt cameras.
Conventional zoom lens systems known so far for use on 35-mm silver salt cameras are broken down into two types, one designed for a single-lens reflex camera such that light passing through a taking lens is guided to a finder subsystem, using reflective means such as a quick return mirror or half mirror attached to the rearmost end of the zoom lens system and the other for a zoom compact camera such that an optical subsystem used for a finder alone is used independently of a taking lens.
The first-mentioned type system for a single-lens reflex camera enables an image approximate to an actual image on film to be confirmed through a finder subsystem, because the finder subsystem makes use of light passing through a taking lens. In particular, this system has an advantage that parallax can be compensated completely. However, some space is needed for inserting an optical path-splitting reflective means into the rearmost end of the zoom lens system, as mentioned above. In the case of a currently available system using a quick return mirror in particular, an extra space for mirror movement and retraction is required in addition to that needed for passing rays therethrough. More illustratively, a 35-mm single-lens reflex camera is required to have an air space of usually about 38 to 40 mm on the final lens surface. This constitutes one of grave barriers on reducing the length from the foremost end to film surface of the zoom lens system.
In the second-mentioned compact camera type of zoom lens system in which the finder subsystem is independent from the taking lens, there is no need of inserting an optical path-splitting reflective means into the rearmost end of the lens system, unlike the single-lens reflex camera system mentioned above; in other words, it is possible to reduce the length of the taking lens. However, it is deprived of the merit that an image approximate to an actual image on film can be confirmed through the finder subsystem. Of course, this merit is among those of the single-lens reflex camera system. In particular, a severe demerit is the occurrence of parallax.
In order to prevent the occurrence of parallax and thereby provide a solution to the problems mentioned above, zoom lens systems in which the initial portion of the finder subsystem passes through the taking lens have so far been known from JP-A-61-114217 and U.S. Pat. No. 4,838,668.
However, the system disclosed in JP-A-61-114217 makes no appreciable contribution to compactness, because the beam splitter that defines an optical path-splitting means remains fixed and the subsequent lens units again remain fixed; in precise parlance, the zooming action of the zoom lens is born by a space located therein, that has so far been located at the rearmost end of the taking lens, rather than by the lens units located subsequent to the beam splitter. As disclosed in the first example of JP-A-114217, the lens length at the focal length at the wide angle end is indeed 125 mm with a zoom ratio falling short of 3. Thus, the system is in no sense said to be made compact.
The system set forth in U.S. Pat. No. 4,838,668, on the other hand, has a well-enough zoom ratio of 3 to 4 and is made compact in a sense of the shortest lens length being 101 to 115 mm. Since the focal length is located on the wide angle side, however, the focal length at the telephoto end is 95 to 116 mm, and this is insufficient for lenses for moderate telephoto to which general users prefer. This is because the system lends itself suitable for wide angle zooming starting with a first lens unit that is of negative power. For this reason, the system referred to in the example of this specification is satisfactory in terms of the wide angle image-formation performance, but is dark at the telephoto end, as expressed by an F-number of 6 or more and is less than satisfactory in terms of the image-formation performance, especially spherical aberration and coma.